Our invention concerns a method of achieving image registration in a web fed, rotary printing press of the class having a series of print stations for successively imprinting images of different colors, for example, on a continuous web of paper or like material traveling along a predetermined path through the print stations.
The image registration method of our invention finds a typical application in rotary web presses of the type used in the manufacture of business forms, wherein the web travels through a succession of stations such as those for printing, perforating, numbering, punching, slitting, and zigzag folding. An example of this type of printing apparatus is extensively discussed in U.S. Pat. No. 4,177,730 issued Dec. 11, 1979, to Schriber et al. and entitled "Method And Apparatus for Web Printing".
In a multicolor web press the exact registration of the different color images on the web is of utmost importance for the manufacture of high quality products. The conventional practice for such image registration has been to visually examine the relative displacements of the images of registration marks that have been imprinted on test paper by the successive plate cylinders. Any one or more of the plate cylinders have then been adjusted circumferentially and/or laterally to an extent determined by the relative displacement of the registration mark images in the longitudinal and/or transverse direction of the web. We object to this long familiar practice because of the prolonged makeready time and waste of paper involved as it usually requires several test printings before all the plate cylinders come into complete alignment.
We are aware of recent efforts in the printing industry toward the automation of image registration. One known method of achieving image registration in the longitudinal web direction, which has won some limited acceptance in the industry, dictates the optical sensing of registration mark images on the web. The sensor output pulses are compared with reference pulses recurring at the same rate as the revolutions of the plate cylinders. Incremental phase or circumferential adjustment mechanisms for the plate cylinders are activated until the sensor output pulses agree with the reference pulses. We object to this prior art method, too, because it requires the preparation of the reference pulses recurring at the same rates as the revolutions of the plate cylinders, and of the registration marks such that the sensor output pulses will be in synchronism with the reference pulses when all the plate cylinders are in phase. The recurrence rates of the reference pulses, moreover, must be altered each time different diameter plate cylinders are mounted in the machine. As an additional drawback this known method does not lend itself to the lateral adjustment of the plate cylinders.